Do all Bluetooth LE devices implement the HCI interface?

Question

Recently I am studying things related to Bluetooth Low Energy. I understand that the BLE stack can be split into Application, Host and Controller layers. Host and Controller layers can communication with each other via the HCI interface. My question is, in some devices where the Host and Controller are placed on the same SoC, is there a need to still implement the HCI interface?

Answer 1

Many Bluetooth SoC stacks designed for embedded devices do not implement HCI but instead implement something more simple/custom.

For example, Nordic Semiconductor's "soft devices", Dialog Semiconductor's DA1458* devices let their host layer talk "directly" to the link layer.

More recently however, the latest trend seems to have been to shift to HCI anyway, especially for a bit larger devices with tons of flash. This approach is taken by Nordic Semiconductor's nRF Connect SDK, using Zephyr. The host then serialises HCI commands to memory which are then deserialised by the lower layer, without any actual data transfer taking place.

Answer 2

Technically, it is certainly not mandatory to establish the HCI interface, but there are many good reasons to do so. I would like to mention two here.

• The current implementations choose an SDR approach for the RF part.
  Then there are two microcontrollers in the SOC. One that takes over
  the SDR baseband function and a second one that runs the stack and
  the application.
• When it comes to the licensing and regulatory area, it will be necessary to be able to perform specified tests. The tools used there usually also use the HCI interface.